This invention relates to an oscillation compensating circuit for use in an electronic timepiece that includes a device which constitutes a heavy load, such as an illumination lamp or buzzer.
Electronic timepieces ordinarily incorporate such devices as an illumination lamp and buzzer that present a heavy load when activated. The lamp is used to illuminate the liquid crystal display under poor lighting conditions, and the buzzer is employed to provide an audible tone in a timepiece having an alarm function, the buzzer sounding when a preset time has arrived. Activation of the loading device such as the lamp or buzzer draws a level of current which is several thousand times greater than that normally drawn by the timepiece circuitry, and the result is a substantial increase in the voltage drop across the internal resistance of the battery which constitutes the power source of the timepiece. This internal resistance gradually increases with the age of the battery in use and also exhibits an increase at a low operating temperature owing to its temperature dependence. The voltage drop across the internal resistance will therefore assume an even greater magnitude whenever the loading device is activated by an old battery or at a low operating temperature, or both.
A drop in the power source voltage tends to have a greater affect upon the crystal-controlled oscillator circuit of an electronic timepiece than upon its frequency divider and timekeeping circuits, and the battery voltage may in fact drop to such a level that the oscillator ceases to function. This is particularly the case if the loading device is activated when the timepiece battery has aged and the operating temperature is low.
One method which has been previously proposed to overcome this problem has been to supply the oscillator circuit from a booster circuit when the loading device is activated. However, the oscillator circuit can be compensated only if the booster circuit is functioning, and whether this is the case or not depends on its receiving the signal from the oscillator circuit after the signal has been divided down by the frequency divider circuit. In other words, the requisite condition is that the booster is functioning during the activation of the loading device. Thus if the oscillator circuit stops oscillating because the loading device is activated for a prolonged period of time, compensation becomes impossible because the requisite condition no longer holds. The present invention, however, overcomes the foregoing problem through a novel method which is entirely different from that of the prior art.
Specifically, the present invention is directed to an electronic timepiece of the type in which a battery is used to power a crystal-controlled oscillator circuit and a loading device, the oscillator circuit having an inverter and passive elements such as capacitors for effecting oscillation and resistors for limiting current. Use is made of the fact that a decrease in the electrical constants of the passive elements is accompanied by a reduction in the oscillation shut-down voltage of the oscillator circuit, i.e., the minimium value of supply voltage applied to the oscillator circuit which will enable it to sustain oscillation. To this end, the oscillator circuit is provided with selection means operable to change the electrical constants of the passive elements so as to offset the voltage drop described above. The electrical constants are reduced in value by controlling the selection means in conjunction with activation of the loading device. The effect of this reduction is to ensure continued operation of the oscillator circuit by allowing the oscillation shut-down voltage of the oscillator circuit to be lowered whenever the loading device is activated. This ensures that the operation of the oscillator circuit will not be adversely affected by a drop in supply voltage across the internal resistance of the timepiece battery, particularly a very large voltage drop that can be caused by a massive increase in the internal resistance of the battery due to its age or by operation at a low temperature, or a combination of both of these conditions.